Corrosion-resistant and wear-resistant ni-based alloy

ABSTRACT

A raw powder having a composition containing, in weight percent, B: 2.2 to 3.0%; Si: 3.0 to 5.0%; Mo: 18 to 25%; Cu: 1 to 15%, the balance of Ni and unavoidable impurities, with a weight ratio of Mo content to B content being from 7 to 9, is produced using a molten metal spraying method, and the raw powder is then sintered, thereby a corrosion-resistant and wear-resistant Ni-based alloy is produced.

TECHNICAL FIELD

The present invention relates to a corrosion-resistant and wear-resistant Ni-based alloy.

BACKGROUND ART

Needs for fluororesin molded articles such as protection sheets for solar cell modifies and water treatment filters have been increasing year by year. Fluororesin parts are formed into predetermined shapes by using molding apparatus such as an extrusion molding machine or an injection molding machine.

High wear resistance may be required for parts to be put in a molten resin environment of the resin molding machine, such as a barrel, etc. of the extrusion molding machine. For such parts, a sintered Ni-based cermet is used, as described in Japanese Patent No. JP 4121694 B2 (hereinafter referred to as “Patent Document 1”), the patentee of which is the same as the applicant of the present patent application.

However, during molding of the fluororesin, the fluororesin is sometimes decomposed to evolve a corrosive gas (fluorine-containing gas), in which even the Ni-based cermet having inherently high corrosion resistance and wear resistance can be worn early.

To prevent or suppress corrosion loss, it is contemplated to use Hastelloy C (trade mark) as a highly corrosion resistant Ni-based alloy, or CH-501 material supplied from Kubota Co. Hastelloy C is an Ni—Mo—Cr based corrosion-resistant alloy available from Haynes International, Inc. (USA), and is excellent in corrosion resistance, but has a low hardness and inferior in wear (abrasion) resistance. CH-501 is an Ni-based cermet and is characterized by having a fine structure, but it needs sintering by HIP and thus high production cost is problematic. That is, if the aforementioned known materials are used, although loss of material caused by corrosion can be decreased, there are problems that the wear resistance is not sufficient and the working life is short and that the part production cost (for example, barrel manufacturing cost) is increased.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a corrosion-resistant and wear-resistant Ni-based alloy having sufficient corrosion resistance and wear resistance even in a circumstance in which a corrosive gas such as a fluorogas is present.

The present invention is based on the alloy of the Patent Document 1 (JP 4121694 B2) and improves the corrosion resistance without sacrificing the wear resistance mainly by addition of Cu and optimization of a Mo/B ratio.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view for explaining an alloy structure of an alloy according to the present invention.

FIG. 2 is a copy of an electron microscopic photograph (secondary electron beam image) showing the alloy structure of the alloy according to the present invention.

FIG. 3 is a schematic view for illustrating a method of integrating the alloy of the present invention with a substrate material during sintering.

EMBODIMENTS OF THE INVENTION

The alloy of the present invention follows the characteristics of the alloy structure of the Patent Document 1 (JP 4121694B2) that improves the toughness without deteriorating the wear resistance by binding aggregates of hard micro particles aggregated in a spherical or lumpy shape by a metallic binder phase of excellent toughness. In the alloy of the invention, the amount of Mo solid-solubilized in the metal binder phase is increased compared with the alloy of the Patent Document 1 and, in addition, added Cu is solid-solubilized in the metal binder phase thereby enhancing the corrosion resistance of the metal binder phase and, thus, the entire alloy. The enhancement of the corrosion resistance is achieved with less sacrifice in wear resistance than the Hastelloy C.

Specifically, in the alloy of the invention, the entire alloy structure comprises a binder phase (a) in which Si, Mo, and Cu are solid-solubilized in Ni (metal binder phase), and a spherical or lumpy hard material assemblies (b) dispersed in the binder phase (a), and the metal structure of the hard material assembly (b) contains a binder phase (c) in which Si, Mo, and Cu are solid-solubilized and a dispersion phase (d) comprising borides such as Mo₂NiB₂ and Ni₃B dispersed in the binder phase (c) in Ni in the same manner as in the binder phase (a) (refer to FIG. 1 and FIG. 2). In the alloy of the present invention, the size of the hard material assembly (b) is preferably about 30 to 300 μm in the same manner as the alloy of the Patent Document 1 which is a base for the present invention.

The raw powder used for the production of the alloy of the present invention is prepared, for example, by an atomizing method (molten metal spraying method) using a molten metal formed by melting NiB, Si, Mo, Ni and Cu and has a composition containing, in weight percent, B: 2.2 to 3.0%; Si: 3.0 to 5.0%; Mo; 18 to 25%; Cu: 1 to 15%, the balance of Ni and unavoidable impurities, with a weight ratio of the Mo content to the B content being from 7 to 9. An atomized powder having a particle diameter of 30 to 300 μm obtained by sieving atomized powder through a sieve of a predetermined mesh is used as the raw powder. A powder having a metal structure in which hard particles comprising borides such as Mo₂NiB₂ and Ni₃B are dispersed in the binder phase here Si, Mo, and Cu are solid-solubilized in Ni can be obtained by preparing the raw powder of the alloy of the present invention by the atomizing method, and a sintered alloy having a binder phase where Si, Mo, and Cu are solid-solubilized in Ni is obtained by sintering the powder (binder phases (a), (c)), and the sintered alloy (alloy of the present invention) exhibits excellent corrosion resistance. On the contrary, it has been confirmed that when the raw powder is prepared by a milling method, an NiSiMo compound is formed and the corrosion resistance of the sintered alloy is deteriorated if the NiSiMo compound is present. In sintering the raw powder, molding is performed preferably, for example, by a vacuum sintering method or a hot isostatic pressing method.

The composition of the alloy of the present invention is, in weight percent, B: 2.2 to 3.0%; Si: 3.0 to 5.0%; Mo: 18 to 25%; Cu: 1 to 15%, the balance of Ni and unavoidable impurities. Further, the weight ratio of the Mo content to the B content is from 7 to 9. With a view point of enhancing the sinterability, the powder before sintering preferably contains 0.01 to 0.5% by weight of C but addition of C is not always indispensable. In the present specification, all percentages that express the composition or the content mean “% by weight” unless otherwise specified.

Reasons for defining the ingredients described above are to be described individually.

Mo is solid-solubilized in the binder phase (the binder phases (a), (c)) to enhance the corrosion resistance of the alloy. If the Mo content is less than 18%, the amount of Mo solid-solubilized in the binder phase is decreased and no sufficient effect of enhancing the corrosion resistance can be obtained. Further, if the Mo content exceeds 25%, a sintering temperature has to be higher in order to obtain sound sintered products, which increases the production cost.

B forms a boride (Mo₂NiB₂) as hard particles together with Ni and Mo to enhance the wear resistance of the alloy. If the B content is less than 2.2%, the amount of Mo₂NiB₂ formed is decreased, resulting in deterioration of the wear resistance (however, since the amount of Mo solid-solubilized in the binder phase increases, the corrosion resistance is enhanced slightly accordingly). If the B content exceeds 3.0%, Mo solid-solubilized in the binder phase decreases and the corrosion resistance deteriorates unless the Mo content is increased by an amount of the resulting Mo₂NiB₂. However, the sintering temperature has to be higher as the Mo content is increased, which increases the production cost (sintering cost). Accordingly, the B content is defined as 2.2 to 3.0%.

As has been described above, in the alloy of the present invention, added Cu is solid-solubilized in the binder phase in addition to increase of the amount of Mo solid-solubilized in the binder phase, thereby enhancing the corrosion resistance of the binder phase and, further, the entire alloy. As can be seen from the description for the B content described above, Mo contained in the alloy is partially consumed to form Mo₂NiB₂ in accordance with the B content and the remaining Mo is present while being solid-solubilized in the binder phase. In consideration of this, the weight ratio of the Mo content to the B content (Mo/B weight ratio) has to be 7 or more in order to enhance the corrosion resistance to an extent that the enhancement is observed. On the other hand, as described above, the raw powder prepared by the atomizing method has a metal structure in which hard particles comprising the borides such as Mo₂NiB₂ and Ni₃B are dispersed in the binder phase in which Si, Mo, Cu are solid-solubilized in Ni. If a great amount of Mo is solid-solubilized in the binder phase of the powder, this results in a problem that a sintering temperature necessary for obtaining a sound structure becomes higher. Accordingly, the Mo/B weight ratio is defined as 9 or less.

Cu, like Mo, is solid-solubilized in the binder phase to enhance the corrosion resistance of the alloy. If the Cu content is less than 2%, the amount of Cu solid-solubilized in the binder phase is small and no effect of enhancing the corrosion resistance appears. On the other hand, if the Cu content exceeds 15%, Cu type compounds are formed and the corrosion resistance of the alloy deteriorates. Further if the Cu content exceeds 15%, the toughness is lowered tending to cause fine chipping and, as a result, the low wear resistance is deteriorated. Accordingly, the Cu content is defined as 1 to 15%. When the wear resistance is of importance, the Cu addition amount is preferably 10% or less.

Si has a function of lowering the sintering temperature. If the Si content is less than 3.0%, no sufficient effect of lowering the sintering temperature can be obtained. On the other hand, if the Si content exceeds 5.0%, it is not preferred since an NiSi compound that deteriorates the toughness of the alloy and an NiSiMo compound that deteriorates the corrosion resistance of the alloy tend to be formed. Accordingly, the Si content is defined as 3.0 to 5.0%.

C has an effect of reducing an oxide film on the surface of the powder and lowering the sintering temperature of the atomized powder. If the C content (addition amount) is 0.01% or less, the effect of reducing the oxide film on the surface of the powder is small and no sufficient effect of lowering the sintering temperature can be obtained. If the C content is 0.5% or more, carbides are precipitated more and the strength and the high temperature corrosion resistance deteriorate. Accordingly, the amount of C when added is defined as 0.01 to 0.5%. C is added preferably, but the extent of oxidation at the powder surface is sometimes small depending on the production condition of the atomized powder, etc. In such a case, the addition amount of C is decreased as much as possible. Two methods may be considered for the addition method of C: one is a method that adds C when melting the raw material of the atomized powder followed by spraying; the other is a method that melts a raw material not containing C to produce the atomized powder, and then adds C (graphite) to the atomized powder, in the usual case. By either of the methods, oxides at the powder surface are reduced sufficiently by the addition of C and the sinterability of the atomized powder can be enhanced. When C is added when melting the raw material as in the former method, C may be added alone or C may be added by the addition of carbides of Mo, Si, B or the like, and similar effect can be obtained in either of the cases.

The alloy of the present invention can be used suitably to parts, for example, a barrel or a screw of a plastic molding machine in contact with molten plastics (particularly, plastics containing fluorine). Since the alloy of the present invention is relatively expensive, it is preferred to provide only the portion in contact with the molten resin by lining over a substrate material (usually comprising iron and steel material or cast iron), rather than forming the entire part with the alloy of the present invention. The production process is to be described briefly with reference to FIG. 3. FIG. 3 shows a cylindrical body 1, a rod body 2, upper and lower lids 3, and a raw powder 4 filled between the cylindrical body 1 and the rod body 2. In this state, a mold releasing agent is applied to the surface of the cylindrical body 1, or the surface of the rod body 2, and the surface of the lid 3 and sintering is performed at a predetermined temperature to obtain a structure in which the cylindrical body 1 (or rod body 2) and the raw powder 4 (a sintered product made from the raw powder 4) are integrated. The cylindrical body 1 may be, for example, a substrate material (comprising a steel material or a cast iron) of a barrel. Further, the rod body 2 may be, for example, a substrate material of a screw (comprising a steel material). For the configuration of a sintering die or a sintering jig for forming a sintered body (sintered layer) on the surface of the substrate material, those disclosed, for example, in Japanese Laid-Open Publication: JP-A No. JP-4-202705A of the patent application filed by the applicant of the present patent application (and corresponding U.S. Pat. No. 5,336,527 and corresponding German Laid-Open No. DE 4139421A) can be utilized. U.S. Pat. No. 5,336,527 is incorporated herein by reference.

The content of expensive Mo is increased in the alloy of the invention compared with the alloy of the Patent Document 1. However, since Cu which is less expensive to NI is added, the content of expensive Ni is decreased by so much as the Cu addition amount. Accordingly, the material cost is substantially identical with that of the alloy of the Patent Document 1. Further, the processing cost for atomizing the alloy of the present invention is identical with that for the alloy described in the Patent Document 1. Further, since the alloy of the present invention can be produced at a sintering temperature not so much different from that of the alloy of the Patent Document 1 (although slightly higher) and the shrinkage during sintering is not different from that of the alloy in the Patent Document 1, parts can be produced by using an identical production facility. That is, when the alloy of the present invention is used, the parts can be produced by the same total cost as that for the alloy of the Patent Document 1.

EXAMPLE

The present invention is to be described more in details with reference to specific examples.

As shown in the upper section of following Table 1, eight types of specimens of specimens Nos. 1 to 8 were prepared. In the following Table 1, “existent material” means an alloy having the alloy composition of the Patent Document 1 (JP 4121694 B2) and “Mo/B” means a value for Mo content/B content by weight ratio. Further, for each of the specimens Nos. 1 to 4, 7, and 8, C (carbon) is added by 0.1%. “Sintering temperature” in the table means the lowest temperature at which a sound sintered structure free of voids can be obtained and, which was determined by experiment. A corrosion test and a wear test were performed on each of the specimens described above. In the corrosion test, a rectangular test piece of 4×7×25 mm was immersed in a 10% hydrofluoric acid at 50° C. for 24 hours and corrosion loss was measured. In the wear test, abrasion loss was measured for a test specimen comprising a pin of 8 mm diameter by a pin-on-disk wear tester of Takachihoseiki Co. Ltd. under the condition at a load of 1,000 N, a friction velocity of 0.2 m/sec and a friction distance of 400 m. The results are shown in the lower section of the following Table 1.

TABLE 1 Powder Sintering Speci- prepara- Composition (wt %) temper- men tion Mo/ ature No. method Ni B Si Mo Cu B Remarks (° C.) Compara- 1 Atomiza- Bal 3.1 4.6 20.0 — 6.5 Existent 1040 tive tion material Example 2 Atomiza- Bal 2.6 4.1 17.3 10.9 6.7 Existent 1080 tion material + Cu 3 Atomiza- Bal 2.8 4.1 22.5 — 8.0 Composition 1070 tion of invented alloy Different powder preparation method 4 Milling Bal 2.7 4.2 21.1 11.0 7.8 1110 5 (Hastelloy C) — 6 (Kubota CH501) — Example 7 Atomiza- Bal — 4.0 21.3 10.5 7.6 Composition 1100 tion of invented alloy 8 Atomiza- Bal 2.8 4.1 21.5  2.5 7.7 Composition 1080 tion of invented alloy Speci- Barrel men Corrosion loss Abrasion production No. (mg/cm² · hr) loss (g) cost ratio Compara- 1 0.035 0.003 1.0 tive 2 0.020 0.015 1.0 Example 3 0.030 0.006 1.0 4 0.033 0.005 Difficult to produce 5 0.010 0.200 2.5 6 0.003 0.163 3.5 Example 7 0.005 0.015 1.0 8 0.007 0.009 1.0

As can be seen from the lower section in Table 1, the alloys of the present invention (specimens Nos. 7, 8) have corrosion resistance to hydrofluoric acid (index of corrosion resistance to fluorine gas) which is greatly enhanced compared with the existent alloys (specimens Nos. 1, 3), and also have corrosion resistance equivalent with that of the existent alloys used so far (specimens Nos. 5, 6) for the portion at which the problem of corrosion loss took place. The wear resistance tends to lower somewhat in the alloys of the invention (specimens Nos. 7, 8) compared with the existent alloys (specimens Nos. 1, 3), but the wear resistance is greatly enhanced compared with the alloys used so far (specimens Nos. 5, 6) for the portion at which the problem of corrosion loss took place. That is, it can be seen that improvement in the corrosion resistance to hydrofluoric acid can be attained in the alloy of the present invention while suppressing the deterioration of the wear resistance to a minimum level.

When the alloys of present invention (specimens Nos. 7, 8) are compared each other, the specimen No. 7 of high Cu content is somewhat excellent in the corrosion resistance but somewhat inferior in the wear resistance.

In the lower section of Table 1, “barrel production ratio” shows the production cost for producing a barrel of a resin extrusion molding machine in terms of a ratio assuming the case in which the alloy of the Patent Document 1 (existent material) is used as a reference (=1). The specimens Nos. 7, 8 as the alloy of the present invention can be produced at a production cost equal with that for the existent material. The alloy of specimen No. 4 having the composition identical with the alloy of the present invention but produced by a different powder production method cannot be joined to the iron and steel substrate material simultaneously with sintering or joint thereof is extremely difficult because of large shrinkage during sintering (that is, not practical as an industrial production method). In the alloy of the present invention, a raw powder (atomized powder) prepared by the molten metal spraying method in the same manner as the alloy of the Patent Document 1 (existent material) is used and such a raw powder causes less shrinkage during sintering and can be joined easily with the iron and steel substrate material simultaneously with sintering. Therefore, a barrel coated with the corrosion resistant and wear resistant Ni-based alloy can be produced at a low cost. Further, the alloys of specimens Nos. 5, 6 are difficult to join to the iron and steel substrate material simultaneously with sintering, or require a high manufacturing cost since special sintering method (HIP) is necessary. 

1. A corrosion-resistant and wear-resistant Ni-based alloy obtained by producing, using a molten metal spraying method, a raw powder having a composition containing, in weight percent, B: 2.2 to 3.0%; Si: 3.0 to 5.0%; Mo: 18 to 25%; Cu: 1 to 15%, the balance of Ni and unavoidable impurities, with a weight ratio of Mo content to B content being from 7 to 9, and then sintering the raw powder.
 2. The corrosion-resistant and wear-resistant Ni-based alloy according to claim 1, wherein the powder before sintering further contains 0.01 to 0.50% of C on the weight %.
 3. A part of a resin molding machine in which at least a portion in contact with a resin is formed of the corrosion resistant and wear resistant Ni alloy according to claim
 1. 4. A part of a resin molding machine in which at least a portion in contact with a resin is formed of the corrosion resistant and wear resistant Ni alloy according to claim
 2. 